1. Field of the Invention
The present invention relates to a steering device for a vehicle of a power steering device or a steer-by-wire system, especially the steering device for the vehicle having a reaction actuator to act a reaction force on a steering wheel or other steering member.
2. Description of the Related Art
It is well known that there is the electrical power steering device to assist steering operation by applying the steering force created by an electrical motor to the steering mechanism. The conventional electrical power steering is equipped with a controller to drive the electrical motor according to a steering torque applied to the steering wheel by a driver to achieve the adequate steering assist force according to the steering torque.
There is also well known a steer-by-wire system having no mechanical connection between the steering wheel and the steering mechanism by detecting a steering angle of the steering wheel by a sensor and transmitting the detected steering angle by a wire to generate the driving force to an actuator for steering according to an output from the sensor, as shown in a published patent applications of Tokkai 2001-191937 or 2003-63434. The steer-by-wire system can change freely a ratio of steered angle of steered wheel (herein called as a steered ratio) against the steering angle of the steering wheel. It can achieve a stable control of a vehicle motion by controlling the steering angle by the actuator for steering independently to an operation of the steering wheel.
There is other steering device for the vehicle to be able to change the steered ratio than the steer-by-wire system, for example, a variable gear ratio steering system to be able to change a rotation transmission ratio between the steering wheel and the steering mechanism, as shown in a published patent applications of Tokkai 2006-2805 or 2004-42829.
Since there is no mechanical connection between the steering wheel and the steering mechanism in the steer-by-wire system, a steered reaction force of a self-aligning torque from the steered wheel is not transmitted to the steering wheel. Therefore, it is equipped with a reaction actuator to transmit the reaction force to the steering wheel. The reaction actuator is controlled based on a steering angle and a steering torque applied to the steering wheel. This is executed by setting a target steering torque based on the steering angle and controlling in a feedback way the reaction actuator in order to coincide an actual steering torque with the target steering torque.
For this feedback control in the conventional steering device for the vehicle, a torque sensor is equipped to detect the steering torque applied to the steering wheel. An analog torque signal from the torque sensor is input into an analog-digital input port of a micro computer and the micro computer calculates the target steering torque based on the steering angle and controls the reaction actuator by receiving a feedback signal from the torque sensor.
However, since a range of the input voltage in the analog-digital input port of the micro computer is predetermined, a range of the detected torque should be wide so that it forces to make a wide range of the output variation from the torque sensor. Therefore, it makes a worse resolution of the detected torque thereby to make a control-ability of the reaction actuator worse and to restrain an improvement of a steering feeling.
In detail, it should be transmitted a stroke end feeling to the driver by acting the large steering reaction force to the steering wheel around a maximum steering angle. And also it should be transmitted a small reaction force to the steering wheel around a neutral position of the steering angle because a self-aligning torque is small. Therefore, the micro computer should be set all range of the output from the torque sensor to the range of the input voltage of the analog-digital input port so that the resolution of the torque signal should be low after it is transformed to the digital signal.
Instead of the steer-by-wire system, one of the power steering devices or the variable gear ratio steering system can be installed the reaction actuator to control the steering reaction force. However, the same problem to the steer-by-wire system should be happened in these devices or systems.